In 1064 the Seljuk Turks, under their sultan Alp Arslan, invade Armenia - for many centuries a disputed frontier region between the Byzantine empire and neighbours to the east. Alp Arslan follows his success here with an attack on Georgia, in 1068. These acts of aggression prompt a response from the Byzantine emperor, Romanus IV Diogenes.

The armies meet in 1071 at Manzikert, near Lake Van. The battle, a resounding victory for the Seljuks, is a turning point in the story of the Byzantine empire. Within a few years there are Turkish tribes in many parts of Anatolia. Some of them are bitter enemies of the Seljuks, but the Seljuks are now the main power in this borderland between Islam and Christianity.

Emotive appeals: 1095-1096

In March 1095 the pope, Urban II, is holding a council of his bishops at Piacenza. It is attended by ambassadors from Constantinople. When the pope allows them to address the assembly, they make a passionate appeal for Christian soldiers to come and help the Byzantine emperor, Alexios I, in his struggles against the Turks. Everyone is impressed.

This seems to have been a seminal moment in inspiring the pope with an ambitious new idea. In November of the same year, at another council (this time at Clermont), Urban takes the issue a crucial step further. Word is spread that on November 27 he will make a great announcement. A platform is set up in a field outside the city gate. A large crowd assembles. The pope rises to address them.

He describes a pitiful state of affairs in the holiest places of Christendom, now in the hands of the Turks. Pilgrims are persecuted and even prevented from reaching the shrines. He urges Christians, rich and poor alike, to march east to recover Jerusalem. He contrasts the poverty and squabbling of life in Europe with an image of a promised land of prosperity and peace. He even offers a new inducement - a plenary indulgence to all who take part. (An indulgence saves the sinner some of the pain of Purgatory, with a plenary one promising greater remission.)

Crusaders should be ready, says the pope, to leave their homes in the late summer of 1096 (after the harvest is in). They should make their way to Constantinople, where a Christian army will assemble.

Constantinople and western Christians: 1097-1099

By April 1097 the various crusading groups are ready to advance together from Constantinople. They are accompanied at this stage by a Byzantine army, for the first task is to clear a route through Anatolia - recently seized from the Byzantines by the Seljuk Turks.

The immediate target, close to hand, is the Seljuk capital in the heavily fortified Byzantine town of Nicaea. After a siege it is taken, in June, and restored to the Byzantine emperor. This is virtually the last act of cooperation between the crusading armies of western Christians and the Greek emperor in Constantinople.

The first crusade recovers northwest Anatolia for the Byzantine empire, but the rest remains in Turkish hands. During the next century troubles multiply for Constantinople, particularly in its relations with western Europeans. The Normans of south Italy and Sicily, profoundly hostile to eastern Christianity, frequently raid the shores and islands of Greece. Antioch, beyond Anatolia, is also in Norman hands - after the leaders of the first crusade betray their agreement with the Byzantine emperor, allotting this ancient Greek city to the Norman Bohemund as a hereditary principality.

But there are even more intense hostilities at closer quarters, among merchants competing bitterly for trade in Constantinople.

Venice and Constantinople: 1082-1201

During the 12th century Venetian merchants make excellent use of the exclusive trading privilege granted them in the Byzantine empire, in 1082, for their help against the Normans. But their wealth and arrogance provoke profound hostility in Constantinople.

In an attempt to curb them, the emperor makes trading agreements with Genoa in 1169 and with Pisa in 1170, following this in 1171 with the confiscation of the goods of every Venetian merchant in the empire. In 1182 the people of Constantinople take matters into their own hands with a massacre of the Latins (or Roman Catholics) living in the city.

Dynastic conflicts in the last years of the 12th century compound the troubles of the Byzantine empire, and accidentally play into the hands of Venice. In 1195 the emperor Isaac II is deposed and blinded by his brother. Isaac's son, Alexius, is imprisoned. In 1201 he escapes and makes his way to western Europe to seek assistance in the recovery of his throne.

In the same year arrangements are being made in Europe for yet another crusade, the fourth, to the Middle East. This time it is proposed that the crusaders depart in a great fleet from Venice. The Venetians, masters of secret diplomacy, suddenly have all the necessary threads in their hands. They weave them into an intricate and profitable web of deceit.

The fourth crusade: 1202-1204

Inspired by the pope's preachers to set off for the east, a new wave of crusaders makes travel arrangements in Venice in 1201. Their immediate target is Egypt, now thought to be the most vulnerable part of Saladin's empire in the eastern Mediterranean.

Venice drives a hard bargain. The city will provide ships for 4500 knights and their horses, 9000 squires to serve them and 20,000 foot soldiers; food for a year for the entire expedition; and fifty galleys as an escort. For this the crusaders will pay 85,000 silver marks and will cede to Venice half of any lands they conquer. This is agreed, with a departure date planned for some time after June 1202.

Venetian diplomats immediately get in touch with the sultan in Egypt, with whom they have excellent trading agreements, to assure him secretly that Venice will not allow the crusading fleet to reach his shores. Behind the scenes the doge is also negotiating with agents of Alexius, son of the deposed emperor in Constantinople. It seems possible that the crusaders might be diverted to this rich and ancient city, where Venice by now has several grudges to settle.

Soon the hard facts of commerce are playing into Venetian hands. The crusading army is assembled in Venice by the summer of 1202. But it has nowhere near assembled the agreed sum of 85,000 silver marks.

The Venetians propose a compromise. They will accept deferred payment and yet honour their side of the bargain, if the crusading army will do them a small favour on the journey out to Egypt. Venice has for a while been disputing control of Dalmatia with the king of Hungary. The Hungarians have recently seized an important coastal city, Zara (now Zadar). It would be a fine thing if the crusaders would recover this city.

The crusaders sail from Venice on November 8 and arrive at Zara on November 10. They besiege the city for five days and pillage it for three. It is then decided that it is too late in the year to continue eastwards. They make a winter camp.

During the winter the Venetians agree terms with Alexius. If placed on the throne in Constantinople, he will pay Venice the sum owed by the crusaders. He will also provide funds and men to help the crusade on its way.

The proposal is put to the crusading army and with some reluctance is accepted. The fleet reaches Constantinople in June 1203. The crusaders break through the great chain protecting the harbour and breach the city walls in July. On August 1, in Santa Sophia, Alexius is crowned co-emperor - alongside his blind father. With the immediate purpose achieved, the crusade should be able to continue on its way. But now it is Alexius who cannot deliver his side of the bargain.

The sack of Constantinople: 1204

The crusaders camp outside Constantinople while Alexius, as emperor, tries to raise his debt to the Venetians by taxing the citizens and confiscating church property. For nine months growing resentment within the city is matched by increasing impatience outside. In April the Venetians persuade the crusaders to storm Constantinople and place a Latin emperor on the throne. For the second time they succeed in breaching the walls.

The doge of Venice and the leading crusaders instal themselves in the royal palace. The army is granted three days in which to pillage the city.

The Venetians, from their long links with Constantinople, can appreciate the treasures of Byzantium. They loot rather than destroy. St Mark's in Venice is graced today by many rich possessions brought back in 1204 - parts of the Pala d'Oro, the porphyry figures known as the tetrarchs, and above all the four great bronze horses.

The crusaders, mainly French and Flemish, are less refined in their tastes. They tend to smash what they find. They ride their horses into Santa Sophia, tear down its silken hangings, destroy the icons in the silver iconostasis. A prostitute, placed on the patriarch's throne, obligingly sings a bawdy song in Norman French.

Latin and Byzantine empires: 1204-1261

The seizure of Constantinople by the fourth crusade does wider damage than the physical destruction of the city. The Byzantine empire is fatally weakened by the crusaders who now grab fiefs for themselves in Greece and western Anatolia (following the example of the first crusaders, a century earlier, in Palestine and Syria).

A Latin empire is set up in Constantinople on the same feudal principle as the Latin kingdom of Jerusalem. One of the crusaders, Baldwin, Count of Flanders and Hainaut, is elected emperor. On 16 May 1204 he is crowned in Santa Sophia. Later in the year he distributes lordships to his vassals.

The Byzantine court survives, in a much reduced state, in the ancient city of Nicaea. From this base, over the next three generations, Byzantine emperors gradually push their Latin opponents westwards out of Anatolia. Eventually, in alliance with Venice's bitter enemy, Genoa, they succeed in 1261 in recovering Constantinople.

Byzantine emperors rule in Constantinople for almost another two centuries. But they are isolated, weak, and in the end helpless against encirclement by a new and powerful enemy - the Ottoman Turks.